Sound transducer system



Feb. 5, 1963 J. P. JONES, JR

SOUND TRANSDUCER SYSTEM Filed Sept. 5, 1961 JOHN PAUL JONES JR.

ilnired rates Patent 3,076,370 SQUND TRANSDUCER SYSTEM John Paul Jones, In, Wynewood, Pa, assignor to Newgation Computer Corporation, a corporation of Pennsyivania Filed ept.5, TQiiLEer. No. 135,930 8 (Jlaims. (Ci. 179-1) This invention relates to sound transducer systems, particularly of the type that can be used for medical diagnosis purposes.

Conventional electronic stethoscopes, of the type used for medical diagnosis purposes, suffer from i by physicians I several major disadvantages; For'one; tne coupling ethciency between the sound source and the mechamcal diaphragmof the sound pickup device is low. For this reason, outside noise interference often obliterates the sound from the source. In addition, the low frequency response of the conventional systems cannot be optimized because of their mechanical inertia. Since the low frequency sounds are of paramount interest, this is a 1 the sound pickup transducer system,-particularly of the purposes, where the These and further objects of the invention are achieved 1 by a sound transducer which employs a thin membrane which contacts the surface (such as the body of a patient) of a sound source. The membrane. also serves to enclose a fluid sound transmission medium within the transducer casing. This fluid, in combination with the membrane, provides a uniform sound conducting medium between the sound source and a pressure sensitivecapacitor, which is also housed within the trans ducer casing.

pacitor of the tunedcircuit of an oscillator. frequency of the oscillator sound. These frequency variations are detectedby a frequency discriminator circuit to provide an output volt age.

The novel features that are of the invention are set forth Thus, the

considered characteristic inthe appended claims.

The transmitted sound causes variations in the capacitance of the capacitor, which is thecavaries in response to the 1 The invention'will be best understood, however, from the following description when read in connection with the accompanying drawing, in which:

FIGURE 1 is a side view, in sectiomof a sound transducer embodying the invention;

FIGURE 2 is a sectional view taken along the line 2-2 of FIGURE 1; and

FIGURE 3 is a schematiccircuit diagram of a tran-' sistor oscillator circuit and frequencydiscriminator circult for use in a sound transducer system embodying the i invention.

Referringnow to the drawing, FIGURE 1 is a side elevational view, in section, of a sound transducer Jembodying the invention, while FIGURE 2 is a sectional view, taken along the line 2--2 of FIGURE 1. The sound transducer includes a casing 5, of approximately 2 inches in diameter, andmade of a molded material such as rubber or plastic, to which is joined, by suitable cement, .a printed circuit board 6." The portion of the transducer which, when the transducer is used for medical diagnosis purposes,

is placed contact with the body of a patient, comprises a thin membrane 8,which is made of a flexible material, such as Mylar. The membrane 8 is fastened securely in place by two members 10 and 12, which are placed in locking engagement with the casing 5. The purpose of the membrane 8 is to hold a liquid 14 in the lower third of the transducer, between the printed circuit board 6 and the membrane 8. The liquid 14- preferably has a specific density equal to the specific density of the human body. Silicone is an-exarnple of a liquid which has been found to be satisfactory for thispurposeo Other liquids could be used, however.

The sound pickup portion of the transducer comprises a pressure sensitive capacitor, having an air dielectric. t

One plate ofthe capacitor comprises a thin metal plate 16 of a conductive material, such as copper. The capacitorplate 16 is soldered to the under side of the printed circuit board 6 and is connected, by. a solder connection (not shown) to the appropriatecomponcnts of theprinted circuitry, as will become apparent from a consideration of FIGURE 3. The second plate ofthe pressure sensitive capacitorcomprises a membrane 13, which is coated with a thin layer of a conductive metal, such as silver. rated from the first place 16 by a spacer 2a of suitable insulating material. The direct current level of thepiclup is maintained, in part, by a series of concentric .smail air vents in the capacitor plate 18, as shown in FIGURE 2. The apparatus on the under side of the printed circult board 6 is completed by a pressure barrier diaphragm 22, which is mounted on a secondspacer 24, which separates the pressure barrier diaphragm 22 from the capacitor plate or diaphragm 18.

The capacitor comprising the metal sheet 16 and the diaphragm lfiis a variablecapacitor in the tank circuit i The emilter 32 of the transistor 26 is connected to a tap on the lIlClllC,Ol 33, thus.

completing a feedback circuit between the base of the transistor 26. Bias voltages are supplied to the transi tor 25 by means of a battery 45, thepositive terminal of which is grounded. The ne ative ter o n a mural of the battery lil is connected through a resistor 42 the emitter and to the collector 3i thus providing a reverse bias voltage t for the collector 3%). The negative terminal of the battery 40 is also connected through a resistor 44 to the base 28, thus maintaining the with respect to the base 28.

Output signa's fromthe transistor oscillator circuit are derived from the collector 3i by means of a coupling capacitor 46 and the primary winding 48 of a transformer 50. The center-tapped secondary winding 52 of the transformer 50' is the input to a frequency discriminator circuit of conventional design.

The frequency discrimirator circuit inclu es a pair of onpos'tely poled diode rectifiers 54 and 56 and an inductor 58 which is connected frm the ary winding 52 to the juncion of'a pair of capacitors 6i) and62, which are connected it arywinding52. A p ir of output terminals 64 and 65 are provided, one of which is connected to the high voltage The second capacitor plate 18 is sepa- 28, collector 3i), and emit er 32 i The variable capacitor of FIGURE 1 One plate w.th an inductorfid, compleing emitter 32forwardly biased center tap of the se'ondparlle with tl e secondi aovaevo terminal of a load resistor 66, and the other of Which is grounded. The other terminal of the load resistor 66 is grounded. The circuit is completed by a capacitor 68, which is connect: from the jun:tion of the capacitors 6i) and 62 to a center tap on the load resistor 65.

The components of the oscillator and frequency discriminator circuits of FIGURE 3 are not shown in EEG- URES l and 2, for simplcity. They wculd be mounted on printed circuit boards, however, which in turn would be connected to the printed circuit board 6.

The thin membrane 8 provides a very thin reactive area when placed in direct contact with the sound source. Contact wi.h the body of a patient can be enhanced by wetting the membrane 8. The membrane 8, in combination with the fluid 14, which is held in place within the transducer by the membrane 8, provides an almost perfect uniform sound conducting medium from the sound source to the capacitor plate 13. The resonant frequency point of the membrane 8 and the fluid 14 is very low so that the resonant effects of the transducer are negligible. Because of this construction, sound travels directly from the sound source until it impinges on the broad surface of the membrane 18, which is one plate of the pressure sensitive capacitor. The efficiency of sound transmission between the source and membrane 18 is very great, especially compared with conventional sound transducers. Moreover, the damping eflect of the fluid 14 excludes the interference of external unwanted sounds to a large degree.

In operation, sound from the source travels through the liquid 14 and places pressure on the capacitor diaphragm 18. The diaphragm 18 thus flexes an amount proportional to the intensity of the sound. Movement of the diaphragm 18 changes its distance from the first capacitor plae 16, causing a change in the capacitance of the capacitor comprising the plate 16 and the diaphragm 18. Variation of this capacitance, since it is the capacitance of the transistor oscillator tuned circuit (FIG. 3) varies the frequency of the oscillator circuit. The variation in frequency is detected by the frequency discriminator circuit. Accordingly, the voltage at the output terminals 64 and 65 of the discriminator circuit varies in accordance with variations in the oscillator frequency and, hence, with variations in the amplitude of the sound from the sound source. A capacitance change of, for example, 1 micromicrofarad will cause a voltage change of 1 volt at the output terminals of the discriminator cIrcuit. Accordingly, with a capacitor plate spacing of ten thousandths of an inch to provide a 100 micromicrofarad capacity, the pressure sensitive diaphragm 18 would have to move only ten millionths of an inch relative to the capacitor plate 16 to provide a useful output voltage at the discriminator output terminals of one tenth of a volt.

The invention described herein provides highly efi'icient coupling of sound from a sound source to a sound pickup, especially important to stethoscopes for medical diagnosis purposes. Moreover, interference from external sound sources is minimized, yet the construction is simple and not expensive.

What is claimed is:

1. A sound transducer comprising, in combination, a casing for said transducer including a thin flexible diaphragm member, pressure sensitive capacitor means mounted within said casing, said capacitor means cmprising a pair of electrodes, one of said electrodes being a relatively rigid metal sheet member and the other of said electrodes being a conductive flexible membrane, and a fluid contained within said casing between said diaphragm and said capacitor means to provide with said diaphragm a uniform sound transmitting medium between a sound source and said capacitor means.

2. A sound transducer comprising, in combination, means adapted to provide contact with a sound source in cluding a thin broad area diaphragm member, pressure sensitive capacitor means comprising a metal sheet electrod-e and a conductive flexible membrane electrode separated by an air dielectric, and means including a fluid pro viding a sound transmission medium between said diaphragm member and said capacitor means to provide variation in the capacitance of said capacitor means 1n response to sound from said source.

3. A sound transducer comprising, in combination, arr

oscillator circuit, tuning means for said oscillator circuit including a pressure sensitive capacitor, said capacitor comprising a metal sheet electrode and a conductive membrane electrode separated by an air dielectric means for engaging said transducer in sound transmission contact with a sound source including a thin diaphragm, means providing a fluid sound transmitting medium between said diaphragm and said capacitor to vary the capacitance thereof and the frequency of said oscillator circuit in re sponse to sound waves, and a frequency discriminator circuit connected with said oscillator circuit to provide a voltage output in response to said variations in the frequency of said oscilaltor circuit.

4. A sound transducer for a medical stethoscope com prising, in combination, a casing for said transducer including a thin flexible diaphragm member for contacting the body of a patient, pressure sensitive capacitor means including a conductive flexible membrane mounted within said casing, and a fluid having a specific density approximately equal to the specific density of the human body contained within said casing between said diaphragm and said capacitor means to provide with said diaphragm a uniform sound transmitting medium between said body and said capacitor means.

5. A sound transducer as defined in claim 4 wherein said fluid is silicone.

6. A sound transducer comprising, in combination, an oscillator circuit, tuning means for said oscillator circuit including a pressure sensitive capacitor, said capacitor having a first rigid plate and a second flexible plate, means for engaging said transducer in sound transmission contact with a sound source including a thin flexible diaphragm, means providing a fluid sound transmitting medium between said diaphragm and said capacitor to vary the pressure on said second plate, the capacitance of said capacitor, and the frequency of said oscillator circuit in response to sound waves, and a frequency discriminator circuit connected with said oscillator circuit to provide a voltage output in response to said variations in the frequency of said oscillator circuit.

7. A sound transducer as defined in claim 6 wherein said oscillator circuit includes a tank circuit and said pressure sensitive capacitor is the variable capacitance of said tank circuit.

8. A sound transducer comprising, in combination, a transistor oscillator circuit including a tank circuit mounted on a printed circuit board, tuning means for said oscil lator tank circuit including a pressure sensitive capacitor, said capacitor including a first rigid conductive plate mounted on said printed circuit board and a second flexible metal coated membrane plate mounted in close spaced relationship to said first plate, means for engaging said transducer in sound transmission contact with a sound source including a thin flexible diaphragm, means providing a fluid sound transmitting medium between said diaphragm and said capacitor to vary the pressure on said second plate, the capacitance of said capacitor, and the frequency of said oscillator circuit in response to sound waves, and a frequency discrminator circuit connected with said oscillator circuit to provide a voltage output in response to said variations in the frequency of said oscillator circuit.

References Cited in the file of this patent UNITED STATES PATENTS 

8. A SOUND TRANSDUCER COMPRISING, IN COMBINATION, A TRANSISTOR OSCILLATOR CIRCUIT INCLUDING A TANK CIRCUIT MOUNTED ON A PRINTED CIRCUIT BOARD, TUNING MEANS FOR SAID OSCILLATOR TANK CIRCUIT INCLUDING A PRESSURE SENSITIVE CAPACITOR, SAID CAPACITOR INCLUDING A FIRST RIGID CONDUCTIVE PLATE MOUNTED ON SAID PRINTED CIRCUIT BOARD AND A SECOND FLEXIBLE METAL COATED MEMBRANE PLATE MOUNTED IN CLOSE SPACED RELATIONSHIP TO SAID FIRST PLATE, MEANS FOR ENGAGING SAID TRANSDUCER IN SOUND TRANSMISSION CONTACT WITH A SOUND SOURCE INCLUDING A THIN FLEXIBLE DIAPHRAGM, MEANS PROVIDING A FLUID SOUND TRANSMITTING MEDIUM BETWEEN SAID DIAPHRAGM AND SAID CAPACITOR TO VARY THE PRESSURE ON SAID SECOND PLATE, THE CAPACITANCE OF SAID CAPACITOR, AND THE FREQUENCY OF SAID OSCILLATOR CIRCUIT IN RESPONSE TO SOUND WAVES, AND A FREQUENCY DISCRIMINATOR CIRCUIT CONNECTED WITH SAID OSCILLATOR CIRCUIT TO PROVIDE A VOLTAGE OUTPUT IN RESPONSE TO SAID VARIATIONS IN THE FREQUENCY OF SAID OSCILLATOR CIRCUIT. 